Beginner Topic 230 of 1102 States of Matter & Solutions
Avogadro's Law
Equal volumes of gases at same temperature and pressure contain equal numbers of molecules.
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In-depth explanation of Avogadro's Law
Avogadro's law states that at constant temperature and pressure, the volume of a gas is directly proportional to the number of moles, so V/n is constant. Equal volumes of different gases therefore contain equal numbers of molecules under the same conditions. This relationship helped establish that diatomic gases such as hydrogen and oxygen are made of molecules rather than single atoms. It also explains why gases react in simple whole-number volume ratios, which Gay-Lussac observed experimentally. The law is one of the foundations of stoichiometry for gaseous reactants and products.
Going deeper: Avogadro's Law
Key ideas: Use V1/n1 = V2/n2 at fixed T and P. It gives the molar volume as the same for all ideal gases under the same conditions, about 24.5 L at room conditions. In reactions between gases, volumes can be used directly as mole ratios, so 1 L of N2 reacts with 3 L of H2 to give 2 L of NH3 under constant T and P.
Common mistakes and connections: A mistake is applying volume ratios to liquids or solids, or to gases at different temperatures or pressures. Another is equating equal masses with equal volumes. Avogadro's hypothesis is about molecules, not atoms, which is why 1 L of H2 holds the same number of molecules as 1 L of He. Links to the mole concept and the ideal gas law.
Visualization of Avogadro's Law
At fixed T and P, volume is proportional to moles: equal volumes of any gas hold equal numbers of molecules, 22.4 L per mole at STP. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Two litres of hydrogen react with one litre of oxygen to form two litres of steam, a 2:1:2 volume ratio that matches the mole ratio in 2H2 + O2 -> 2H2O.
Frequently asked questions about Avogadro's Law
What is Avogadro's Law?
Equal volumes of gases at same temperature and pressure contain equal numbers of molecules.
Is Avogadro's Law a beginner, intermediate or advanced topic?
Avogadro's Law is a beginner topic in States of Matter & Solutions. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Avogadro's Law?
Study these earlier topics in States of Matter & Solutions first: Boyle's Law, Charles's Law, Gay-Lussac's Law.
What comes after Avogadro's Law?
The next topics in States of Matter & Solutions are Combined Gas Law and Ideal Gas Law.
Can I practise Avogadro's Law with quizzes?
Yes. The interactive lesson includes a self-check question on Avogadro's Law, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Avogadro's Law fits in States of Matter & Solutions
Avogadro's Law is topic 11 of 109 in States of Matter & Solutions.